This invention relates to a method and apparatus for measuring the rotation of a corrective contact lens.
The characteristic of astigmatism as applied to the eye itself is typically corrected by an asymmetric prescription of the corrective lens, which is called a toric contact lens. If the amount of astigmatism is known, there are known ways of manufacturing the asymmetric lens in order to cause the focus to occur on the retina. Frequently, however, difficulty occurs in the accurate determination of the prescription for the eye of a particular person having astigmatism, and wishing to wear contact lenses.
To aid in this determination, contact lens manufacturers are known to create tiny, almost invisible markings such as dots and/or lines on the lens, located at the edges of the convexly curved lens. These markings can be formed by known techniques. With placement of the lens on the cornea of the eye, a care giver such as an optometrist, an oculist, or an ophthalmologist, estimates the amount of lens rotation on the eye, usually from the horizontal or from the vertical, and prescribes according to the effect the rotation has on the person's astigmatism, the prescription being for a corrective toric contact lens. Such estimates are too often not accurate enough, resulting in under-correction or over-correction.
There are two degrees of astigmatism to address, 1) the location in degrees of the astigmatism in the patient's spectral prescription, and 2) the amount in degrees of rotation of the toric contact lens. The examination lens of the care provider provides the 1) location between 0 and 180.degree.. When the angle 2) is determined, it has to be added to or subtracted from the prescription for astigmatism. That is, if the contact lens rotation is clockwise, it is called "left add" or "LA," and if it is counterclockwise, it is called "right subtract" or "RS." This 2) rotation is caused by eyelid pressure and gravitational pull, sometime aided by prism ballast of the lens.
Techniques and devices have been proposed for more accurately ascertaining the degree of lens rotation, but these are typically considered too complex, or difficult or inaccurate to use, and therefore are not believed to be widely employed by care givers.